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| Hydrocarbon machine |
•
Magoon
and Dow (1994) : PETROLEUM SYSTEM is
a natural system that encompasses a pod of
active source rocks and all related oil and gas and which includes all the
geologic elements (source, reservoir, seal, overburden rocks) and processes
(trap formation, generation-migration-accumulation) that are essential if a
hydrocarbon accumulation is to exist. The essential elements and processes must
occur in time and space.
•
Petroleum system : elements + processes
•
elements :
petroleum source rock, reservoir rock, seal rock, and overburden rock.
•
processes :
trap formation, generation-migration-accumulation of petroleum, and
preservation of accumulation.
Extent of
Petroleum Sytem
•
Geographic :
a line that circumscribes the pod of active source rock and includes all the
discovered petroleum shows, seeps, and accumulations that originated from that
pod
•
Stratigraphic
: include rock units or essential elements within the geographic extent (source
rock, reservoir rock, seal rock, overburden rock)
•
Temporal :
geologic time of elements and processes of petroleum system
•
Petroleum System Name
Source – Reservoir (degree of certainty)
•
Degree
of certainty
(!) =
proven (geochemically)
(.) =
hypothetical
(?) =
speculative
•
Examples
:
Pematang – Sihapas (!)
Tuban – Kujung (.)
Gumai – Muara Enim (?)
Source Rock
Geochemistry
•
Definition
of source
•
Kerogen
•
Organic
preservation in sediments
•
Source
rock depositional environments
•
Source
rock characterization
Source Rock
•
What
kind ?
•
How
rich ?
•
How
mature ?
Roles of
Hydrocarbon Source Rocks
•
Petroleum
is generated from organic-rich sediments (source rocks) containing organic
matter originating from biological materials. During burial of sediments, the
increase in temperature results in a series of geochemical reactions which
leads from biopolymers to geopolymers, often collectively called kerogen, which
are precursors of petroleum.
•
The
amount, type and composition of petroleum generated is dependent upon the
nature of the organic matter in the source rock and its maturity governed by
its time/temperature history.
Kinds of
Source Rocks
•
Active source rocks : a volume of rock that has generated or is generating and expelling
hydrocarbons in sufficient quantities to form commercial oil and gas
accumulations. The contained sedimentary organic matter must meet minimum
requirement of organic richness, kerogen-type and organic maturity.
•
Spent source rocks : a volume of rock that has generated, possibly a long time ago, its
hydrocarbons and now contains thermally altered organic matter.
•
Potential source rocks : a volume of rock that has the capacity to generate hydrocarbons in
sufficient quantities to form commercial oil and gas accumulations, but has not
yet reached the state of minimum hydrocarbon generation because of insufficient
organic maturation.
Kerogen
•
Kerogen
(from kerosene generator) is defined as the organic component of
source rocks that is insoluble in common organic solvents and aqueous alkali
(NaOH solution). The soluble portion of the organic matter is termed bitumen
or total soluble extract (TSE)
•
Kerogen
is of complex biological origin; it is derived from dead organisms whose
organic remains survive the early stages of diagenesis and lithification. This
biological origin is frequently apparent when kerogen is analysed by
microscopic or chemical techniques. It is derived from the lipid, lignin,
protein, and carbohydrate portions of organisms.
Source Rock
Depositional Environments
•
Lacustrine
source rocks
– freshwater lakes
– saline lakes
•
Paludal
source rocks – freshwater marshes
•
Paralic
source rocks – marine-influenced, salt marshes
– siliciclastic paralic source rocks
– carbonate paralic source rocks
•
Deltaic
source rocks
– upper delta plain (freshwater delta
top)
– lower delta plain (brackish-saline
delta top)
– pro-delta
•
Marine
source rocks
– enclosed restricted basins
– continental shelves
– continental slope and rise
Preservation
of Organic Matter
•
The
principal control on organic richness is the efficiency of preservation of organic
matter in sedimentary environments.
•
Three
factors affect the preservation (or destruction) of organic matter :
§ the concentration and nature of
oxidizing agents
§ the type of organic matter deposited
§ the sediment-accumulation rate
Of these, oxidizing agents are
probably the most crucial factor.
Factors
Enhancing Preservation
•
Stagnant
basins : density stratification with O2-poor bottom waters
•
Oxygen-minimum
layer (OML) : the rate of oxygen consumption exceeds the rate of oxygen influx
•
Restricted
circulation : presence of shallow and deep silling, coal swamps (poor water
circulation, high influxes of organic matter, diminished bacterial activity.
Source Rock
Characterisation
•
For
a source rock, the characterisation is designed to test :
– its richness
– the type of petroleum it is likely
to generate
– its maturity
•
Techniques
/ analyses for characterisation include :
– TOC (total organic carbon)
– Rock-eval pyrolysis
•
Source
potential of S1 (P1), S2 (P2), S3 (P3)
•
Tmax ºC
•
Hydrogen
Index (HI)
•
Oxygen
Index (OI)
•
Production
Index (PI)
•
Potential
Yield (PY)
– Visual examination of kerogen
concentrates
– Extract analysis
– Maturity evaluation (SCI, VR)
– Gas chromatography analysis
– GC-MS analysis
– Carbon isotope analysis
Rock-Eval
Pyrolysis
•
S1
(P1) (ppm) : free HCs released when furnace temperature is 250ºC
•
S2
(P2) (ppm) : HCs cracked from kerogen when furnace temperature is 550ºC
•
S3 (P3) (ppm) : carbon dioxide released during
early stages of pyrolysis
•
T
max ºC : maximum temperature of S2
•
HI
(hydrogen index) : S2/TOC (mg/g) or ratio of released HCs to organic carbon
content
•
OI
(oxygen index) : S3/TOC (mg/g) or ratio of released carbon dioxide to organic
carbon content
•
PI
(production index) : S1/S1+S2
•
PY
(pyrolysis yield) : S2 (ppm) or total of HCs released during cracking of
kerogen compared to original weight of rock
•
Source
potential : S1+S2
•
Tmax,
HI, and OI are each functions of both maturity and kerogen type.
Reservoir
Rocks
•
A
subsurface porous and permeable rock body in which oil and/or gas is stored
(Tver & Berry, 1980).
•
For
a rock to act as a reservoir it must have pores to contain the oil or gas
(porosity), and the pores must be connected to allow the movement of oil and
gas (permeability).
•
A
petroleum play is defined initially by the depositional or erosional limit of
its gross reservoir unit.
Porosity
and Permeability
•
Porosity
: amount of void space in a rock (% voids per bulk volume). Reservoir porosity
affects the reserve of a prospect or play.
•
Permeability
: ability of a rock to transmit fluid through pore spaces. Reservoir
permeability affects the rate of petroleum flow during production.
•
There
is no necessary relation between porosity and permeability. A rock may be
highly porous and yet impermeable if there is no communication between pores. A
highly permeable sand is usually highly porous.
 |
| Depositional environments : deposition of reservoir sediments |
Sandstone
and Carbonate Reservoirs
•
The
primary porosity and permeability of sandstones are dependent on the grain
size, sorting and packing of particulate sediments. Many siliciclastic
reservoirs have a strong diagenetic overprinting that modifies the depositional
porosities and permeabilities (like presence of authigenic clay minerals in the
pore space will reduce porosity).
•
Carbonate
reservoirs are characterized by extremely heterogeneous porosity and
permeability on a number of scales. These heterogeneities are dependent on the
environment of deposition of the carbonate facies and on the subsequent
diagenetic alteration (dissolution, dolomitization, fracturing,
recrystallization, cements).
Glacial
Environment
•
Environments
characterized by deposits on continents, in lakes or in seas, resulting from
the melting of ice masses.
•
Glacial
deposits do not constitute potentially good reservoir rocks. This is related to
the, generally, high amount of fine materials (silt and clays) present in the
deposits.
Alluvial
Fan Environment
•
A
continental environment characterized by coarse sediments, shaped like an open
fan, deposited by an emerging mountain stream with an outlet into a plain or
broad valley.
•
Alluvial
fan deposits are not generally reservoir rocks for petroleum because they fail
to connect laterally to source rocks, do not contain good source rock facies,
are not sufficiently extensive laterally, do not have proper seals, have low
permeability and porosities.
Desert
Environment
•
A
continental environment characterized by deposits resulting from wind action
(aeolian). Three aeolian subenvironments : dune, interdune, sand sheet.
•
Aeolian
deposits are complex, heterogeneous reservoirs due to : lateral discontinuity,
impermeable and permeable alternations, various permeabilities and related
textural changes causing low transmissivity across laminae, isolated reservoir.
Braided
Stream Environment
•
A
continental environment characterized by deposits resulting from a river system
of an interlaced network of low sinuousity channels.
•
Braided
river deposits may constitute potentially good reservoir rocks up to 30 %
porosity and permeabilities of thousands of millidarcys.
Meander
Stream Environment
•
A
continental environment characterized by deposits resulting from a river system
of high sinuousity channels generated by a mature stream across its flood plain
on a gentle slope.
•
Meandering
river deposits may constitute potentially good reservoir rocks up to 30 %
porosity and permeabilities of thousands of millidarcys, but they are laterally
restricted. They often contain their own source rocks (plant debris, peat,
lignit, coal).
Delta
Environment
•
A
transitional environment characterized by sediments that have been transported
to the end of channel and deposited at the margin of the standing water (lake,
sea, ocean).
•
Deltaic
sands have generally good reservoir rocks up to 35 % porosity and
permeabilities of thousands of millidarcys in mouth bar deposits, the
permeabilities are still good. Due to general coarsening upward, reservoir
qualities are better developed towards the top; this is contrary with fluvial
deposits which are fining upward. Deltaic reservoirs are being close proximity
to potential sources. Growth faulting is common, structural and stratigraphic
traps are abundant.
Shallow
Marine Siliciclastic Environments
•
Environments
characterized by detrital deposits in moderate water depth (10-200 m), or on
nearshore continent, under tides, waves, wind, longshore currents, or storms as
dominant sediment-moving forces. They include deposits such as : estuarine,
tidal flats, intertidal sand bars, storm deposits, barrier islands, beach
ridges, shorelines.
•
Sand
bodies have, generally, good reservoir characteristics. Their volumes depend on
each depositional facies
Shallow
Water Carbonate Environments
•
Environments
characterized by carbonate deposits generated by biochemical processes in
shallow water (< 100 m).
•
Carbonate
rocks can have good reservoir characteristics depending on the importance of
diagenetic effects. When dissolution has occurred, the porosity and
permeability are very high. Other diagenetic effects reduce the porosity. The
permeability is often related to the presence of fractures which occur
frequently in such rocks. Carbonate reservoirs can be very thick and have a
large extension. Source rocks are often close to the reservoir rocks. Cap rocks
are composed of either shale or anhydrite beds.
Deep Sea
Clastic Environment
•
Environments
characterized by sediments deposited in a large body of water below the action
of waves, resulting from sediment gravity flow mechanisms.
•
Due
to the general immaturity of the sands, their characteristics are often
moderate to poor. The permeability increases from distal to proximal fans.
Distal sands constitute sheet-like beds with no vertical permeability. Proximal
sands can be thick, with good vertical permeability, with a shoestring shape.
Overpressures are often observed.
SEALING
ROCKS
Roles of
Topseal
It
influences migration routes taken by petroleum fluids as leaving petroleum
source rock (laterally- or vertically-focused migration system). Geographic extent of seal rocks defines the effective limits of the
petroleum system.
Definition
and Class of Seal Rock
•
Seal
Rock = rock that has pore throats too small and poorly connected to allow the
passage of hydrocarbons (Downey, 1994).
•
Sealing
= restriction to secondary migration (Allen and Allen, 1990).
•
Two
important classes of seals occur in a petroleum system : (1) regional seals
that roof migrating hydrocarbons and (2) local seals that confine accumulations
(Ulmishek, 1988).
Factors
Affecting Caprock Effectiveness (1)
•
Lithology
•
Ductility
•
Thickness
•
Lateral
Continuity
•
Burial
Depth
Factors
Affecting Caprock Effectiveness (2)
•
Lithology :
caprocks need small pore sizes, so the vast majority of caprocks are
fine-grained clastics (clays, shales), evaporites (anhydrite, gypsum, halite)
and organic-rich rocks.
•
Other
lithologies such as argillaceous limestones, tight sandstones and
conglomerates, cherts and volcanics may also seal, but they are globally less
important, and are frequently of poor quality and geographically of limited
extent.
•
Caprocks
of world’s giant oil fields : 60 % shales, 40 % evaporites; world’s giant gas
fields : 66 % shales, 34 % evaporites (Grunau, 1987)
Factors
Affecting Caprock Effectiveness (3)
•
Ductility :
capability of being stretching without breaking
•
Ductile
lithologies are less prone to faulting and fracturing than brittle lithologies.
•
Ductility
is a particularly important requirement of caprocks in strongly deformed areas
such as fold and thrust belts.
•
A
high kerogen contents appears to enhance the ductility of shale caprocks. Many
source rocks, therefore, also serve as seals.
•
Ductility
is also a function of temperature and pressure. Ductility generally can enhance
in HTHP condition.
Factors
Affecting Caprock Effectiveness (4)
•
Thickness :
a thick caprock substantially improves the chances of maintaining a seal over
the entire prospect.
•
Thin
caprock may have sufficient capillary pressure to support a large HC column (a
clay shale with a particle size of 10-4 mm will have capillary
pressure of 600 psi – Hubbert, 1953, theoretically can hold an oil column of
3000 ft); but thin caprocks, however, tend to be laterally impersistent.
•
Thicknesses
greater than 50 ft are generally required for effective seals (Sluijk and
Nederlof, 1984).
•
A
Thick seal is important and beneficial, but is does not directly influence the
amount of HC column can be held. Where traps are created by fault offset of reservoirs,
thickness of the top seal can be important.
Factors
Affecting Caprock Effectiveness (5)
•
Thickness :
typical caprock thicknesses range from tens of metres to hundreds of metres
(Grunau, 1987). 30 m-thick Ahmadi shales seal 74 BBO Burgan Field (Kuwait), 20
m-thick Arab CD anhydrite seal 80 BBO Ghawar Field (Saudi Arabia).
•
For
gas reservoirs, a thick caprock reduces the risk of substantial losses by gas
diffusion.
Factors
Affecting Caprock Effectiveness (6)
•
Lateral Seal Continuity : in order to provide regional seals, caprocks need to maintain stable
lithological character (and hence capillary pressure and ductility
characteristics) and thickness over broad areas.
•
Most
prolific petroleum provinces in the world contain at least one of the regional
seals.
•
The
lateral variability of the regional seal may be studied using wireline logs and
seismostratigraphic analysis.
•
Some
depositional environments and basin settings are more conducive to the
eastablishment of thick and effective regional caprocks than others.
•
Geographic
extent of seal rocks defines the effective limits of the petroleum system.
Factors
Affecting Caprock Effectiveness (7)
•
Burial Depth of Caprocks : The present burial depth of caprocks does not appear to be important
factor in influencing seal effectiveness.
•
Seals
may be effective at all depths. However, we know that shale pore diameters do
decrease with burial, particularly over the first 2 km.
•
Many
shallow oil accumulations occur in structures that have undergone significant
uplift, bringing well-compacted caprocks close to the surface. The maximum
depth to which shale caprocks once attained (maximum attained depth of burial)
likely to have an influence on sealing capability.
•
3.9
BBO recoverable Duri Field is sealed by Telisa shales positioned at present
depth of only 100 m.
Seal Rock
Analysis
•
Many
stratigraphic horizons have properties of a seal; it is important to identify
those that define the hydrocarbon migration (above the mature source rocks and
regionally extensive) and accumulation system (have seal-transmission couplet)
at the critical moment. All other seals are irrelevant to the petroleum system.
•
Maps
of the distribution, character, and structural attitude of regional seals are
important in understanding the petroleum system.
•
Seal
Potential : (1) seal capacity, (2) seal geometry, (3) seal integrity (Kaldi
& Atkinson, 1993)
•
Seal
capacity : the calculated amount of HC column height a lithology can support
•
Seal
geometry : the structural position, thickness and areal extent of the lithology
•
Seal
integrity : rock mechanical properties such as ductility, compressibility,
propensity for fracturing
For a seal to be truly effective, it needs to
be relatively thick, laterally continuous, relatively homogeneous, and fairly
ductile (Downey, 1984).
TRAPS
Trap
Definition and Roles
•
A
Trap is any geometric arrangement of rock that permits significant accumulation
of oil or gas, or both, in the subsurface (North, 1985; Biddle & Wielchowsky,
1994)
•
The
petroleum exploration industry is primarily concerned with the search and
recognition of trap.
Trap in
Petroleum System
•
A
trap is part of petroleum system. Trap is built by geometric arrangement in a
variety of ways of the two critical components : the reservoir and the seal.
•
The
hydrocarbon-forming process and the trap-forming process occur as independent
events and commonly at different times. But the two process should be in
harmony for trap to contain hydrocarbons.
Trap :
Fundamental Components
•
To
be a viable trap, a subsurface geometric feature must be capable of receiving
hydrocarbons and storing them for some significant length of time. This
requires two fundamental components : a reservoir rock in which to store
the hydrocarbons, and a seal to keep the hydrocarbons from migrating out
of the trap (Biddle & Wielchowsky, 1994)
Trap :
Critical Timing of Development
•
Not
only must a good reservoir and a sealed trap geometry be present for the
existence of a petroleum trap, but the timing of its development must also be
considered.
•
The
trap must be present prior to the petroleum charge in order to trap
petroleum. A trap that developed too late to receive a petroleum charge
will be dry. Thus, an understanding of the history of individual trap growth
together with the burial and thermal history of the basin, is essential to the
evaluation of petroleum prospects.
Trap
Classification
•
Traps
are classified into structural, stratigraphic, and hydrodynamic
traps (Allen and Allen, 1990).
•
Structural
traps : are those caused by tectonic, diapiric, gravitational, and compactional
processes.
•
Stratigraphic
traps : are those whose their geometry are essentially inherited from the
original depositional morphology of, or discontinuities in, the basin-fill, or
from subsequent diagenetic effects.
•
Hydrodynamic traps : are those formed by the movement of interstitial fluids through
basins.
Stratigraphic
Trap
•
As
structural traps of any size are becoming fewer and fewer, except in frontier
basins, an increasingly large proportion of the worlds’s undiscovered resources
are likely to be found in stratigraphic traps.
•
Classification
of stratigraphic traps :
q Depositional : related to
sedimentary facies changes
q Unconformity : either above or below
unconformity surfaces
q Diagenetic : mineral diagenesis,
biodegradation of petroleum (tar mats), phase changes to petroleum gas (gas
hydrates), interstitial water (permafrost)
•
The
detection of stratigraphic traps requires a high level of geological expertise.
Great emphasis must be placed on an understanding of the stratigraphic
evolution of the basin, through a detailed sequence-by-sequence analysis. Of
particular importance is the understanding of palaeogeography and sedimentary
facies for each sequence and sub-sequence.
GENERATION,
MIGRATION, AND ACCUMULATION
Hydrocarbon
Generation
Effects of
Maturity on Organic Matters
•
The
major changes to organic matter that occur with increasing maturity include
three stages of evolution : diagenesis, catagenesis, metagenesis.
•
Diagenesis
: convert organic debris derived from living organisms into kerogen,
temperature < 100 °C, mediated mostly by bacteria
•
Catagenesis
: Thermally degrade kerogen into petroleum,
– temperature 100-150 °C breakdown of kerogen to oil
– temperature 150-230 °C breakdown of kerogen to gas
•
Metagenesis
: generation from kerogen is complete, internal change of the residual kerogen to
graphite, temperature > 230 °C
Petroleum Generation
and Expulsion
•
The
generation of petroleum from kerogen proceeds via a complex series of reactions
during which many types of bonds are broken as a result of thermal stress.
•
The
depth interval in which a petroleum source rock generates and expels most of
its oil is called the oil window. Most oil windows are in the temperature range
from 60 to 160ºC (140-320 ºF). Gas windows are in the 100 to 200 ºC (212-392 ºF) temperature range. From one-half
to two thirds of thermogenic gas comes from the thermal cracking of previously
formed oil.
Mechanics
of Expulsion
•
Expulsion
is also known as primary migration.
•
The
most likely mechanism of expulsion appears to be as a discrete phase through
microfractures caused by the release of overpressure.
•
The
cause of overpressure in the source rock may be a combination of oil or gas
generation, fluid expansion on temperature increase, compaction of sealed
source rock units, or release of water on clay mineral dehydration.
•
The
conversion of kerogen to petroleum results in a significant volume increase.
This causes a pore pressure build up which is sometimes large enough to result
in microfracturing. This release pressure, and allows the migration of
petroleum out of source rock into adjoining carrier beds, from which point
secondary migration processes take over.
Hydrocarbon
Migration
•
Hydrocarbon
migration (secondary migration) concentrates subsurface petroleum into specific
sites (traps) where it may be commercially produced.
•
If
a trap is disrupted at some time in its history, its accumulated petroleum may
re-migrate either into other traps, or leak to the surface.
•
The
main driving forces for secondary migration :
– buoyancy : caused by the difference
between oil (or gas) and the pore waters of carrier beds
– pore pressure gradients : which
attempt to move all pore fluids (both water and petroleum) to areas of lower
pressure.
•
The
main resisting forces for secondary migration :
– capillary pressure, which increases
as pore size become smaller when capillary pressure exceeds the driving forces,
entrapment occurs.
Migration
Pathways
•
Petroleum
will tend to move perpendicular to structural contours.
•
Petroleum
flow may be split when encountering a low, and concentrated along regional
highs.
•
The
geometry of the kitchen also affects petroleum charge volumes; prospects locaed
close to the ends of strongly elongate source kitchens will receive relatively
little charge.
•
Sealing
faults may deflect petroleum flow laterally.
•
Nonsealing
faults allow petroleum to flow across the fault plane into juxtaposed permeable
units at a different sratigraphic level.
Faults and
Hydrocarbon Migration
•
Fault
zones can act as both conduits and barriers to secondary migrtion. The material
crushed by the frictional movement of the fault, the fault gouge, is frequently
impermeable and does not allow the passage of petroleum. Clay smeared along
fault planes also blocks petroeum migration.
•
Fractures
formed in either the footwall or hangingwall, if they remain open, may form
effective vertical migration pathways. This may occur in the uplifted
hangingwalls of compressive faults on release of compressive stresses.
Tensional fractures in the crestal zones of anticlinal structures may also
allow migration of petroleum.
•
Lateral
migration will tend to be inhibited by the presence of faults, since they
interrupt the lateral continuity of the carrier bed.
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